The need for higher data rates has put significant demands on optical communications. Requirements on network performance continue to increase and component specifications have become more stringent. Optical networks employing coherent detection receivers have considerable advantages over direct detection receivers for high data rate communications. One type of coherent detection used to achieve higher bit rate communications is quadrature carrier modulation. Data to be transmitted are used to modulate an in-phase signal and a quadrature signal for each of two orthogonal polarizations of an optical signal, enabling data rates that are four times the data rates of many other modulation schemes for similar network configurations.
The ability to successfully transmit data from an optical transmitter to an optical receiver through an optical communications channel can be impaired by errors introduced at the transmitter. For example, a quadrature phase shift keying (QPSK) modulator typically includes two Mach-Zehnder (MZ) modulators biased at 90° with respect to each other. Any variation from the 90° bias is referred to as transmit quadrature error and can result in degradation of the communications system performance. Although bias control can be used to reduce quadrature error, and to control gain and gain balancing, bias control can introduce errors based on nonlinearities in the drive controls and chirp in the MZ modulators. Such errors can be difficult and expensive to detect at the optical transmitter.
Typically, each MZ modulator is biased at extinction at its half wave voltage VΠ and is driven by a data signal having a binary excursion of 2VΠ to produce a binary phase shift keying (BPSK) signal. Any error in the bias voltage Vbias applied to a MZ modulator to achieve extinction results in a DC tone in the transmitted optical signal. This error is referred to as carrier leakage and degrades the optical signal to noise ratio (OSNR).
The two BPSK signals generated in a QPSK modulator are ideally time-aligned so that there is no time difference between the data signals applied to the MZ modulators. Sampling phase errors occur if the BPSK signals are not time-synchronized to within a small fraction of the data rate of the signal applied to the two MZ modulators. Sampling phase error leads to degradation of the bit error rate (BER) at the optical receiver.
Quadrature error, carrier leakage, IQ timing error and the difference in the DC gain of the I and Q channels (IQ gain error) at the optical transmitter generally vary in time due to environmental changes. Detection of these errors at the optical transmitter is difficult and requires expensive and complex implementations.
The present invention addresses the need to reduce impairments to the optical signals based on the transmitter errors identified above and provides additional advantages.